However, the results won't become official until more than 8,500 additional ballots are tabulated in 10 days: those from provisional voters and absentees mailed in from overseas and the military. Once that final total is calculated, Ohio law requires a mandatory recount if the margin is with 0.5 points.

Several media organizations were holding off calling the race, and O'Connor did not concede.

"We always knew this was going to be a close race, and while we don't know the results quite yet, I know that this campaign left it all on the field," he said in a statement.

"No matter what happens next, I'm proud to stand beside the thousands of volunteers who have made this campaign possible."

President Donald Trump took credit for the apparent 0.9 percent win by the Zanesville state senator, tweeting:

"When I decided to go to Ohio for Troy Balderson, he was down in early voting 64 to 36. That was not good. After my speech on Saturday night, there was a big turn for the better. Now Troy wins a great victory during a very tough time of the year for voting. He will win BIG in Nov."

In his victory speech, Balderson credited O'Connor with running a good race, and said he looked forward to running against him again in November.

Ohio Republican Chairman Jane Timken said in a statement: "Tonight Troy Balderson and the constituents of Ohio's 12th Congressional District have sent a message to Democrats and media pundits across the country. America is on the right track under President Trump and Republican leadership, and the so-called 'blue wave' is nothing more than wishful thinking."

Upper Arlington Rep. Steve Stivers, head of the GOP's congressional campaign committee, said, "Nancy Pelosi and her liberal donors flooded this district with money and ads in an attempt to buy this seat. However, Troy's focus never wavered from the issues central Ohioans care about like fighting opioids, creating jobs, and lowering taxes."

Balderson won by roughly 1,700 votes, or 0.8 percentage points.

The the night has progressed just as forecast: The Democrat would roll up a big margin from votes cast before Election Day, and Balderson would draw closer throughout the evening.

The final pre-election polls had showed the race virtually a dead heat.

Ohio native Kyle Kondik of the University of Virginia Center for Politics said the district should have been an easier win for the GOP. The district, he said, "contains some of the most bedrock Republican turf in the state and this basically could have went either way."

"The GOP likely keeps the seat but the result is very much in line with what we've seen all over the country in special elections this cycle: Democrats often running well ahead of Hillary Clinton's 2016 performance," he said. "I don't think this was a particularly impressive showing for Republicans."

Licking County has completed its tabulation; Balderson won with 61 percent of the final, unofficial vote — a margin of more than 8,000 votes over O'Connor.

Not only did O'Connor win a huge early vote victory in his home Franklin County, he achieved a 21-point win in key Delaware County, meaning many unaffiliated and GOP voters backed the Democratic Franklin County recorder in the special election.

Franklin, Delaware and Licking make up 80 percent of the 12th district's vote, meaning Balderson had to prevail by large margins among votes cast on Election Day to win.

"The Republican better see a significant turnaround in Delaware County same day voters or this baby is over," tweeted Gov. John Kasich's top political adviser, John Weaver of Texas.

But it appears that's happening as results pour in from the district President Donald Trump won by 11 points in 2016.

The two major parties spent nearly $8 million on the campaign, with the Paul Ryan-affiliated super PAC the Congressional Leadership Fund dumping some $3 million into the race, including deploying a shadow campaign operation that knocked on 500,000 doors.

And in a demonstration of GOP nervousness about the district, Trump, Vice President Mike Pence, House Speaker Paul Ryan and Majority Leader Kevin McCarthy campaigned and/or raised money in the district

Absentee votes are always counted first. O'Connor took home 80 percent support from Franklin County voters who cast ballots before Election Day, giving him an immediate 8,300-vote lead out of the box over Balderson.

A gathering of roughly 100 Balderson supporters at the DoubleTree Hotel in downtown Newark was generally upbeat, but a big deficit in early returns somewhat muffled the enthusiasm level. They picked up as results going across the Fox News ticker showed the race virtually tied.

Balderson "worked his tail off," said Rep. Scott Ryan, R-Newark, but so many outside groups bring their messages into the race. "A candidate's campaign can't control all that. It makes it a little different animal than what I'm used to," said Ryan, who runs the Ohio House Republican campaign committee.

Asked if the close race was a worry as he looked ahead to November, Ryan said, "Yes, but a special is a different animal. Turnout is always important, but in a special, turnout is everything. The November election is a different deal because you'll have more of the electorate engaged."

But, considering that Republicans have held this district for so long, Ryan said, "the fact that this is close is certainly not what we were hoping for."

The O'Connor election night watch party had the energy of a victory party well before results were in. A packed Ohio Civil Service Employees Association headquarters was busting at the seams with more than 300 supporters. Chants of "Danny" burst out sporadically at an event that felt more like a rock concert than a political event.

Those in attendance cheered with each new graphic flashing on the screen showing O'Connor with even the slightest of leads. With a close race and potentially long night ahead, the O'Connor camp anxiously anticipated each updated vote totals.

"We are coming out of tonight with the wind at our backs. This is a district where normally a Democrat would never be competitive," Rick Neal, Democrat running against Stivers in the adjoining 15th district. "We have won already. To get this close is fantastic."

Regardless of who won, Balderson and O'Connor will face off again in November. Republicans say they think Balderson will perform in the general election because turnout will be higher across the district, and this won't be such a singular focus of Democrats trying to make a statement against Trump.

But the polls weren't even closed yet Tuesday when O'Connor won his first victory.

However, it didn't involve Tuesday's election.

The Franklin County Board of Elections ruled that Matthew Brendan O'Connor, a Libertarian, did not qualify for the November general election ballot. The Libertarian needed five valid signatures from registered Franklin County voters, but he provided only four; one was ruled invalid.

There were worries among Democrats that the "other" O'Connor would draw votes away from the Democratic nominee in the fall election.

The ruling does not affect today's election between O'Connor and Balderson.

Regardless of the special election outcome, which determines who serves the few months remaining on former Rep. Pat Tiberi's term, the two major party candidates will meet again in November to decide who gets the full two-year term.

Trump again tweeted this morning urging voters to back Balderson, a state senator: "Ohio, vote today for Troy Balderson for Congress. His opponent, controlled by Nancy Pelosi, is weak on Crime, the Border, Military, Vets, your 2nd Amendment — and will end your Tax Cuts. Troy will be a great Congressman. #MAGA" Trump also recorded automated calls for Balderson urging voters to support his "friend."

Meanwhile, former Vice President Joe Biden recorded automated telephone calls urging a vote for O'Connor: "Electing Danny to Congress puts us one step closer to taking back the House and making sure we have leaders in Washington who will fight for our values — Ohio values."

Even aged recording star Pat Boone was making robo-calls in the match-up deemed a barometer on Trump's performance — especially since he personally appeared for Balderson — as well as a possible indication of how November congressional mid-terms will shape up.